Field of the Invention
The present disclosure is related to compounds for inhibition of indoleamine-2,3-dioxygenase pathway, in particular salts and prodrugs of indoximod with enhanced pharmacokinetic properties relative to indoximod
Summary of Related Art
Tryptophan degradation into kynurenine is mediated by indoleamine-2,3-dioxygenase (IDO1) expressed by plasmacytoid dendritic cells, placental, epithelial and tumor cells and by tryptophan-2,3-dioxygenase (TDO2) expressed mainly by the liver and tumor cells.
IDO1 plays an important role in the regulation of immune responses by triggering anergy on reactive effector T cells and by modulating differentiation and activation of regulatory T cells (Tregs). From a more general viewpoint, the IDO enzyme is involved in pathway that comprises all proteins that directly or indirectly contribute to modulate the immunosuppressive functions dependent on IDO activity, including proteins that mediate induction of IDO expression, activation of enzymatic activity by reductases, post-translational modifications that regulate activity, protein degradation, and the interpretation and transmission of the signals elicited by low concentrations of Trp and the presence of Trp catabolites [collectively known as kynurenines (Kyns)] including catabolic stress sensors integrated into the General Control Nonrepressed-2 (GCN2) pathway, the Aryl Hydrocarbon Receptor (AhR) pathway, and the mammalian Target Of Rapamycin (mTOR) pathways. This concept of integrated downstream regulatory pathways with IDO at the center has emerged from studies on multiple model systems by many research groups and this notion may be critically important for understanding how the IDO pathway is induced, how IDO exerts downstream effects, and the mechanism of action of IDO pathway inhibitors that target IDO directly or target other components of the IDO pathway [1,2].
Therefore, direct pharmacological inhibition of IDO1 enzymatic activity or inhibition of the upstream factors that activate IDO1 enzyme or inhibition of the downstream effects of IDO1 enzymatic activity should stimulate an immune response by multiple mechanisms that may involve preventing anergy of effector T cells, reactivating anergic effector T cells, preventing the activation of regulatory T cells, promoting phenotypic conversion of Tregs to pro-inflamatory TH17 cells and promoting phenotypic reprogramming of immunosuppressive dendritic cells into immunostimulatory dendritic cells.
For these reasons, numerous enzymatic inhibitors of IDO have been described and are being developed to treat or prevent IDO related diseases such as cancer and infectious diseases. Numerous molecules that inhibit IDO enzymatic activity either as competitive or non-competitive inhibitors have been described in the literature, for example in patent applications WO2012142237, WO2014159248, WO2011056652, WO2009132238, WO2009073620, WO2008115804, WO 2014150646, WO 2014150677, WO 2015002918, WO 2015006520, WO 2014141110, WO 2014/186035, WO 2014/081689, U.S. Pat. No. 7,714,139, U.S. Pat. No. 8,476,454, U.S. Pat. No. 7,705,022, U.S. Pat. No. 8,993,605, U.S. Pat. No. 8,846,726, U.S. Pat. No. 8,951,536, U.S. Pat. No. 7,598,287.
One of the first IDO pathway inhibitors studied in preclinical models has been 1-methyl-DL-tryptophan (1mT), a racemic mixture of enantiomers, which was shown to mediate immune dependent rejection of allogeneic fetuses in mice [3] and immune dependent enhancement of antitumor activity of chemotherapy and radiotherapy [4]. Each one of these enantiomers shows different biological properties. 1-methyl-L-tryptophan (L1mT) has been shown to inhibit IDO1 enzymatic activity (Ki=34 μM, [5]) in cell-free assays using purified recombinant IDO1 enzyme, and in tumor cells treated with INFγ or in tumor cell lines transfected with expression vectors that encode IDO1 under the control of an heterologous promoter, while the D isomer (indoximod) does not inhibit enzymatic activity in these type of assays [6]. Nonetheless, both isomers are capable of restoring T cell proliferation in an MLR assay with IDO+ dendritic cells as the stimulator cells, or in syngeneic antigen-dependent T cell proliferation assays using IDO+ DCs isolated from tumor draining lymph nodes [6]. In this type of assay, where IDO+ DCs are present, T cells do not proliferate. However, inhibition of the IDO pathway by these inhibitors restores the proliferative capacity of T cells. Interestingly, both isomers show different potency in this assay, with indoximod being more potent (EC50=30 μM) than L1mT (EC50=80-100 μM) or the racemic mixture (80-100 μM) [6]. Moreover, despite the fact that indoximod does not show inhibition of enzymatic activity in other types of assays, it shows inhibition of enzymatic activity in this co-culture assay, as seen by reduced Trp degradation and Kyn synthesis.
A somewhat puzzling issue has been the fact that indoximod does not show inhibition of IDO1 enzymatic activity in vitro, but somehow mimics the biological consequences of IDO1 inhibition in vivo or in cell based assays. Experimental evidence from a number of research laboratories points to the conclusion that indoximod is participating in the inhibition of the IDO1 pathway. Several possible mechanisms by which this could be taking place are: 1) inhibition of isoforms of IDO1, 2) inhibition of IDO2, 3) alternative formation of indoximod derived metabolites, 4) racemization of indoximod into L1mT, 5) inhibition of Trp transport, 6) inhibition of the GCN2 pathway by formation of indoximod-tRNA complexes, 7) inhibition of enzymes involved in Trp sensing such as WARS1 or WARS2, 8) alteration of autophagy under conditions of amino acid deprivation induced stress or 9) bypassing mechanisms that inactivate mTOR under conditions of amino acid deficiency [7]. These mechanisms are not necessarily mutually exclusive, and so far are compatible with the current experimental data. Further investigations are needed to elucidate which of these biochemical mechanisms is responsible for the biological activity of indoximod.
The biological activity of indoximod to relieve immunosuppression in vivo and in vitro is supported by studies performed in several laboratories in murine preclinical models. Indoximod has demonstrated activity in the following biological assays:                1. In combination with chemotherapy, indoximod demonstrates antitumor effects in animal models of ectopic melanoma, colon and lung tumors, and in orthotopic and autochtonous breast tumor models. The antitumor effect of indoximod is lost in nude and IDO1-KO mice [6].        2. indoximod can prevent the process of activation of mature Tregs in vivo, and facilitates the in vitro and in vivo trans-differentiation of Tregs into pro-inflamatory TH17-like T cells [8, 9].        3. In tumor vaccination protocols, the combination of two different antitumor vaccines with indoximod was effective in converting a higher proportion of Treg cells into TH17-like T cells, with concomitant antitumor effect [9].        4. In melanoma models, combination of anti-CTLA4 (ipilimumab) and indoximod, results in synergistic antitumor effect [10].        5. In vivo, indoximod was more efficacious as an anticancer agent in chemo-immunotherapy regimens using cyclophosphamide, paclitaxel, or gemcitabine, when tested in mouse models of transplantable melanoma and transplantable (4T1) and autochthonous (mmTV-neu) breast cancer [6].        6. IDO1 has also been implicated in the differentiation of nave CD4 T cells into Tregs, by the combined effect of Trp deprivation and the presence of Trp catabolites, through a mechanism that depends on GCN2 [11, 12]. This conversion is interrupted in vivo in the presence of indoximod.        7. Similarly, IDO+ pDCs have also been implicated in the activation of mature Tregs in vivo, which also required an intact GCN2 pathway in the Treg population. This phenomenom could be prevented by excess Trp or by indoximod [8].        8. In addition to preventing the activation of mature Treg cells, indoximod can mediate the conversion of suppressive FoxP3+ Tregs into pro-inflamatory TH17 cells in vitro and in vivo. This conversion of Tregs into TH17 cells required the presence of antigen or engagement of B7 in the pDCs, and the presence of functional IDO1 and GCN2 genes in the pDCs. Indoximod was able to mimic the phenotypic consequences of IDO1 or GCN2 gene ablation [9], therefore supporting its role in inhibition of the IDO pathway.        9. Antitumor and immunologic studies using IDO1-KO mice or pDCs derived from IDO1-KO mice demonstrated that the beneficial effects of indoximod are lost in the context of a genetic background lacking a functional IDO1 [6]. In particular, it was observed that IDO1-KO mice develop tumors, which are not sensitive to treatment with indoximod in combination with chemotherapy. Additionally, pDCs derived from tumor draining lymph nodes of IDO1-KO mice are able to stimulate the proliferation of T cells in culture, to the same extent as IDO(−) APCs. These observations were interpreted as a genetic validation of IDO1 as the pharmacologic target of indoximod. However, this could also be interpreted as indoximod blocking some other point of action within the IDO pathway.        10. The antitumor and immunologic observations made by administration of indoximod were also reproduced by administration of other well documented IDO1 inhibitors (i.e. molecules that inhibit the enzymatic activity of IDO1 in vitro and in cell based assays) such as 5-Br-brassinin, menadione, methyl-thiohydantoin-tryptophan, and analogs of phenylimidazole (unpublished), thereby validating the IDO1 pathway as the pharmacologic target [4, 13, 14].        11. In preclinical animal models, the in vivo pharmacodynamic effects of indoximod are seen mainly in tumor draining lymph nodes, where the effect is seen as activation and proliferation of CD8α+ cells, reduction in the number of FoxP3+ Tregs, reprogramming of Tregs (CD40L−) to immunostimulatory T cells (CD40L+) and reprogramming of IDO+ antigen presenting cells from CD11c+/CD80/86− to CD80/86+ phenotype.        
For these reasons, indoximod is being investigated in human clinical trials for cancer indications. Indoximod is being studied in several cancer indications in combination with different chemotherapeutic and biological immunotherapeutic agents, such as docetaxel, paclitaxel, gemcitabine, Nab-paclitaxel, temozolomide, ipilimumab, sipuleucel-T, or vaccines.
Indoximod is orally bioavailable with a favorable pharmacokinetic (PK) profile (Tmax: ˜3 h; half-life: ˜10 h) and an excellent safety profile. Pharmacokinetic studies in patients have demonstrated that indoximod shows a linear PK profile at doses of up to 800 mg/dose, with maximum plasma concentration (Cmax) of 15 μM and drug exposure (AUC(0-last)) levels of ˜100 μM·h. However, increasing doses above 800 mg/dose up to 2000 mg/dose, does not result in a linear or proportional increase in Cmax or drug exposure, thus potentially limiting the therapeutic activity of this investigational drug.
Mixed-lymphocyte response (MLR) T cell proliferation assay show that T cells that are in an IDO+ environment restore ˜50% of their proliferative capacity at concentrations of indoximod higher than 30 μM. Murine antitumor experiments show that biological effects of indoximod are observed when mice are dosed with indoximod in the drinking water at 3 mg/mL (˜500 mg/kg/day), or dosed orally at 200 mg/kg bid, which results in Cmax higher than 20 μM and exposures greater than 300 μM·h. For these reasons, it is desirable to increase the Cmax and exposure to indoximod in human clinical trials so they may reach the levels necessary for therapeutic activity. However, the non-linear pharmacokinetic profile of this drug makes it unlikely that this could be solved by increasing the dose given to patients.
For the above mentioned reasons we investigated whether different formulation of indoximod such as spray dry dispersions or salts or indoximod prodrugs in different salt forms would increase solubility and absorption rate or reduce blood clearance to levels that increase the maximum concentration and exposure to indoximod. Moreover, we looked for prodrugs and its salts that could result in increases parameters of exposure when dosed orally and in pill (capsule or tablet) dosage formulation.
The results of these investigations showed that a few selected prodrugs resulted in increases in parameters of exposure; and that increases in in vitro solubility and in vivo exposure could be achieved by a few salts of indoximod upon oral administration.